It has been found in the brake system described in patent application Ser. No. 43 19 227 (not published) that the proposed constructive solutions with respect to the arrangement of the annular piston including a controllable restrictor and the details in the solenoid inlet valve, which correspond to the annular piston, require further structural simplification to achieve a most simple miniaturized inlet valve design.
A solenoid valve is disclosed in European patent application No. 0 317 305 which is suitable for use in anti-lock hydraulic brake systems. The solenoid valve has a magnetic core accommodating a coil. The magnetic core is confined by a magnetic armature on one side and by a restrictor member on the other side. The restrictor member has a passage which is closable by a valve needle. The restrictor member is compressed in an axially movable fashion between the magnetic core and a housing cover, in which the pressure fluid inlet is provided, so that in the switching position, where the valve needle closes off the supply duct in the restrictor member, a differential pressure acts on either side of the restrictor member and causes a displacement of the valve needle in relation to the magnetic armature. A compression spring interposed between the valve needle and the magnetic armature is preloaded thereby. The preloading force of the compression spring caused by the difference in pressure on the restrictor member results in a quick release of the magnetic armature from the magnetic core when the electromagnetic excitation is interrupted. This results in short valve opening times. A spring resetting force acting on the valve needle in the opening sense favors the quick opening of the supply duct in the restrictor member. Thus, pressure fluid propagates to an annular slot after having passed through the free passage at the valve needle. The annular slot is provided between the hollow-cylindrical inside wall of the magnetic core and the outside wall of a cylindrical part guiding the valve needle. The fluid which emanates from the valve inlet is conducted through the supply duct in the restrictor member and the subsequent annular slot into a pressure fluid connection which leads to the pressure fluid consumer.
While the circuit configuration of the brake system disclosed in the main application remains unmodified, an object of the present invention is to suggest solutions for miniaturizing the inlet valve which can be achieved at low cost without limiting the operability of the valve.
According to the present invention, this object is achieved by the annular piston, which accommodates the controllable restrictor, which is axially movable within an annular chamber in the valve carrier concentrically relative to the valve closure member of the inlet valve.
Thus, the annular chamber, which originally accommodates an annular filter and the valve closure member, additionally serves for the favorable integration of the annular piston, thereby permitting the external dimensions of the valve carrier and the dimensions of the stepped bore of the valve accommodating member to be maintained. Further, the annular portion between the valve accommodating member and the valve carrier can be used to position the annular filter. A favorable increase of the filter surface for filtering the fluid which, among others, flows between the wheel brake and the non-return cup seal is thereby achieved.